Amphipathic compounds having both hydrophilic and hydrophobic groups within the same molecules spontaneously form various shapes of molecular assemblies in water. Amphipathic compounds form various molecular assemblies in water at the Krafft temperature (TK; referred to as Krafft eutectic temperature, Krafft point, or the like) or higher, as determined depending on the types or concentrations thereof (Non-patent Document 1). Examples of such molecular clusters include closed micelles (e.g., spherical micells and rod micells) with hydrophilic groups oriented outward, closed reversed micelles with hydrophobic groups oriented outward, sponge phases wherein hydrophobic groups or hydrophilic groups of amphipathic compounds are aligned facing each other in bilayer membranes and the bilayer membranes are randomly connected, and various lyotropic liquid crystal phases. Known examples of the lyotropic liquid crystal phase include hexagonal liquid crystal and reverse hexagonal liquid crystal wherein infinitely long cylindrical clusters form a two-dimensional hexagonal lattice, lamellar liquid crystal wherein bilayer membrane sheets are layered at regular intervals in the direction of the Z axis, and cubic liquid crystal having a three-dimensional lattice structure. Amphipathic compounds forming liquid crystal are referred to as liquid crystal compounds.
These molecular clusters, and amphipathic compounds forming liquid crystal in particular, are used for various applications in the fields of cosmetics, pharmaceutical products, and the like. For example, drug delivery systems (DDS) using amphipathic compounds are under active development. Various forms of drug delivery carriers have been produced, including a drug delivery system (Non-patent Document 2) in which a drug is embedded in an intraliposomal aqueous phase or a lipid bilayer prepared from lamellar liquid crystal (Patent Documents 1 and 2). In particular, non-lamellar liquid crystal such as cubic liquid crystal or reverse hexagonal liquid crystal has a high degree of structural stability and is capable of stably retaining various drugs within itself, and thus is attracting attention as a particularly useful drug delivery carrier.
Meanwhile, most forms of cubic liquid crystal found in an amphipathic compound/water system can remain stable only within a narrow concentration range between other phase regions, such as an aqueous micelle solution, hexagonal liquid crystal, lamellar liquid crystal, and reverse hexagonal liquid crystal, which occupy large areas on a two-component (amphipathic compound/water) phase diagram (Non-patent Document 3). Thus, the cubic liquid crystal is used with difficulty as a drug delivery carrier or the like. In recent years, it has been reported that monoacylglycerols including monoolein and phytantriols form “type II cubic liquid crystal” wherein a cubic phase and an aqueous phase are adjacent to each other on a two-component (amphipathic compound/water) phase diagram. It has also been reported that the liquid crystal is relatively stable even when it coexists with excess water. Thus, application of the liquid crystal to a drug delivery system or the like has been attempted (Non-patent Document 4). However, liquid crystal formed by monoolein and the like has low stability at low temperatures. Accordingly, an amphipathic compound capable of forming cubic liquid crystal that exhibits high stability at low temperatures (less than 6° C.) has been developed and the use of the liquid crystal in a sustained release formulation has also been disclosed (Patent Document 3).
However, such liquid crystal compounds stably forming cubic liquid crystal have high viscosity and thus do not allow the compounds to pass through a thin injection needle (e.g., 30 gauge). Hence, these liquid crystal compounds are problematic in that they are used with difficulty as bases for injection formulations.